The author seems to confuse self-learning with learning only the theory.
The real title should be: "Theory without practice is overrated".
And when you word it this way, it's hardly anything surprising.
It's not even that. His assertion seems to be that practice outside of a sufficiently business-like environment doesn't count, because you won't be compelled to focus all of your time and energy on just those things that bring a positive change in rate of value production now. It's a dangerous game to play, but a desirable one moment-to-moment in an environment that severely dislikes disruptive ideas -- the thing that learning for something other than instant gratification is bound to cause.